Not Applicable.
Not applicable.
In the last few years, the number of commercial cellular telephone users has risen dramatically, but the bandwidth allocated to cellular telephony has remained nearly constant. Because of the limited nature of cellular telephony bandwidth as a resource, the cost of obtaining bandwidth has risen dramatically. This necessitates the efficient utilization of available bandwidth resources to maintain commercial viability.
Many intelligent schemes for optimizing the use of available bandwidth resources have been proposed. These methods include such means as signal compression or elimination of non-essential frequency artifacts to reduce the overall bandwidth. Other systems include Time Division Multiple Access (TDMA) where multiple users utilize the same frequency band by transmitting bursts of data in specified periodic time slots or Code Division Multiple Access (CDMA) systems.
The use of Orthogonal Frequency Division Multiplexing (OFDM) as a modulation and multiple access method for commercial wireless communication systems is not widely practiced and is expected to grow in the future. Potential applications include wireless local loop, wireless local area networks and cellular and PCS systems. Possessing many of the benefits of well known time and code division multiple access systems, OFDM based multiple access systems are also referred to as Orthogonal Frequency Division Multiple Access (OFDMA) systems in the literature. Recently, OFDM was chosen as the modulation scheme for the European Digital Audio Broadcast (DAB) standard and the European Terrestrial Digital Video Broadcast (DVB-T) standard. OFDM based hybrid multiple access systems such as OFDM-TDMA and Multicarrier-CDMA are also being actively researched.
Such communication systems consist of a downlink and an uplink. The downlink is the unidirectional communication link from a single base-station (BS) to multiple remote (possibly mobile) transceivers. The uplink is the unidirectional communication link from these transceivers to the BS. Typically, the downlink and uplink occupy distinct non-overlapping frequency bandsxe2x80x94also called frequency division duplex (FDD) operation. It is also possible to operate in time division duplex (TDD) (xe2x80x9cping-pongxe2x80x9d or half-duplex mode) where the uplink and downlink occupy the same frequency band but alternate in time. This is generally preferred only for indoor systems. The uplink is a multiple access channel since the plurality of remote transceivers access or share the uplink channel resources. The downlink can be thought of as a broadcast or multicast link. In general, the problem of interference suppression is more difficult and important for the uplink since typically it represents the capacity bottleneck (compared to the downlink).
One of the major problems faced by wireless communication systems is that of interference. In particular, in OFDM systems, two main categories of interference are Inter-Bin Interference (IBI) and Co-channel Interference (CCI). IBI is the manifestation of loss of orthogonality between different bins of a OFDM system. Each data carrying bin acts as a source of interference (or noise) for every other data carrying bin. CCI refers to any other undesired signal whose spectrum overlaps with the spectrum of the particular OFDM system under consideration and causes interference. For example, sources of CCI may be other analog or digital communication/broadcast systems (which may or may not be using OFDM) operating in the same (or adjacent) frequency band in the same/nearby geographic areas. IBI and CCI can increase the bit-error-rate of the particular frequency bins that are experiencing the interference. As a result, the OFDM system performance may be degraded. Thus, interference suppression techniques are desirable for high-performance systems. A number of different techniques have been prepared to either avoid or suppress interference.
A factor which must be considered in multiple access wireless systems is that of power control or automatic gain control (AGC). Essentially, the receiver must be able to ensure that the received power of each bin is within a certain target range. This problem is made difficult by the presence of fading which can easily cause fluctuations in the received power in the range of 20-40 dB in a matter of seconds. Thus, in wireless systems, some basic power control mechanisms may be used. However, these power control mechanisms may not be perfect. Imperfect power control may exacerbate the effect of IBI.
If CCI is localized in frequency (i.e., narrowband CCI), the particular bin (or bins) that are affected such that the average signal-to interference-plus-noise ratio (SINR) is reduced below a certain threshold can be left unused. If the interference is temporary, the bin can be reused when the SNR improves. The basic procedure is well established in digital subscriber line (DSL) modems which use DMT as the modulation scheme. This procedure may be implemented by the BS in a wireless OFDM system by measuring any CCI across the frequency band of interest. However, the problem is more difficult in wireless systems because of the presence of fading which can also greatly reduce the SNR. Thus, the average SNR must be tracked. Fading results in fluctuations in the channel frequency response with time.
One measure of the rate of change of the channel response with time is given by the so-called Doppler spread (units of Hertz). When there is little relative movement between the receiver and transmitter (or when the propagation environment is relatively static), the multipath fading can be considered to be slow fading and the Doppler spread is around 5 Hz or less (this is not to be confused with the attenuation due to distance which also changes slowly, typically according to the log-normal distribution). In such cases (e.g. wireless local loop and indoor systems), the receiver can track and estimate the channel frequency response for each bin with good accuracy. This is typically accomplished via the use of periodic pilot sub-symbols inserted in the sub-symbol streams of each bin of interest. For example, for a given data carrying bin, every pth (say p=8 or 16) sub-symbol can be pilot (training) sub-symbol to estimate the channel periodically. For in-between sub-symbols, the receiver can estimate the channel by operating in decision directed mode or by interpolation. For fast fading channels (Doppler spread 10-200 Hz), estimating the channel is more difficult and sophisticated time-frequency interpolation techniques must be used (this is a drawback of OFDM).
Several techniques have been proposed in the literature for combating IBI and/or CCI. One method of addressing IBI is to space data carrying bins apart in frequency and leave bins unused there between. This is effective because: (a) the effect of IBI decreases with increase in frequency separation between bins and, (b) for a given total bandwidth, there are fewer active bins. However, this is wasteful of bandwidth and not a preferable solution. Another approach for addressing IBI and CCI is forward error correction (FEC) codes, mostly implemented in conjunction with interleaving. FEC codes may afford some protection against noise and interference. A related method is the use of Trellis coded modulation (TCM) to address IBI and CCI. However, the methods proposed heretofore have met with limited success. A need remains for an improved method and apparatus to overcome the problems associated with IBI and CCI.
One aspect of the present invention is targeted at interference suppression in the uplink of a FDD OFDMA system using spatial signal processing via antenna arrays deployed at the BS receiver. The present invention is not limited to FDD OFDMA, but may be to carry out interference suppression in other scenarios as well such as for hybrid OFDM-TDMA systems: Multicarrier-CDMA systems, TDD systems and in the downlink of the above systems.
The present invention affords a method and apparatus for suppression of CCI via the use of receive antenna arrays at the BS for the uplink channel. In addition, those skilled in the art will recognize that the application of this invention is not limited to the BS uplink channel, but is also applicable elsewhere including the BS downlink and at the remote transmitter/receivers. The intelligent use of antenna arrays for mitigating fading and interference is also referred to as xe2x80x9csmartxe2x80x9d or xe2x80x9cintelligent antennasxe2x80x9d. Smart antenna systems may be carried out through the use of switched beam antennas or adaptive arrays (AA). Switched beam antennas use a fixed beamforming network to provide several output ports corresponding to beams in fixed directions. Signal levels in each beam are monitored and analyzed to switch the beams appropriately among different time or frequency channels depending on the air-interface scheme. Adaptive arrays, on the other hand, electronically steer a phased array by weighting the amplitude and phase of signal at each element in response to changes in the propagation environment. Adaptive arrays provide greater steering flexibility in response to the propagation environment. The preferred embodiment focuses on adaptive arrays. However, switched beam antennas may be used.
A first inventive aspect of the present invention involves an adaptive array (AA) architecture and methods for combating the effect of IBI and CCI over multipath fading channels. As described above, the solution is presented for the uplink of an OFDMA system with synchronous uplink, however, the present invention is not limited to OFDMA. A second inventive aspect involves a method for allocation of frequency bins (i.e., determining the spectral locations or bin numbers) to different users by taking spatial and other information (such as automatic gain control (AGC) information) into account. Each user may require one or more bins to meet a certain quality of service requirement. This aspect of the invention is most appropriate in the context of the adaptive array architecture described above but, in general, is not limited to such a receiver configuration as will be apparent to those skilled in the art.
For multi-element AA receivers, each element may have its own RF-to-baseband conversion and baseband demodulator. Of course, hardware optimizations may be possible. All beamforming and diversity combining algorithms operate on digital complex baseband signals, for instance via general purpose or application specific DSP""s, ASIC""s, in software (such as in software radios) or combinations thereof.
Consider slowly time-varying fading channels (SFC) first. This implies that the channel attenuation coefficients in each frequency bin can be taken to be constant over each symbol (a single complex number). Also, these coefficients may change from symbol to symbol but at a slow rate relative to the symbol rate. Some systems predominantly encounter SFC (e.g. indoor systems) while others encounter FFC""s (e.g. cellular systems along highways). However, most wireless communication systems of interest experience a mix of SFC and FFC conditions.
Generally, wireless communications systems experience two primary types of signal fading within channels, slow time varying fading and fast time varying fading. The IBI due to the time-varying fading nature of a channel is negligible. IBI due to frequency offsets (imperfect synchronization) is still possible. A preferred embodiment of the present invention includes a AA with the elements spaced far apart (5 to 15 wavelengths) to obtain spatial diversity, i.e., independent fading at different antenna elements. The combining method of the preferred embodiment uses maximal ratio combining (MRC) to correct for IBI and Additive white Gaussian noise (AWGN). The MRC is merely a spatial matched filter. If an M element array is used, each bin has a separate M dimensional combining weight vector. To implement MRC, the channel frequency response for each bin may be estimated via periodic pilot sub-symbols. Note that the MRC also subsumes the role of the standard frequency equalization (FEQ) operation.
If CCI is also present, an optimal method (i.e., according to one specific criterion) is the so-called maximum SINR optimum combining (MSOC). This method also uses channel estimation. In addition, some statistics of the signal and interference must also be estimated. Periodic pilot sub-symbols can be used for both these tasks. MSOC has greater computational burden than MRC but greater potential for performance improvement.
For fast time-varying channels, implementing MSOC may require excessive bandwidth overhead for pilot sub-symbols. Basically, if MSOC is implemented on a FFC, it may not give any more benefit than if MRC was used. In fact, the performance can worsen if the channel coefficients are not tracked properly. Unfortunately, the method required to remedy this requires a different receiver architecture than above. Therefore, the receiver architecture has to be chosen to be one of the two and it may not be possible to change it on the fly unless the BS has a flexible software radio architecture. To avoid the foregoing problems, the preferred embodiment proposes the two-stage method as described below.
First, the antenna array is partitioned into sub-arrays. The elements of each sub-array are spaced close together (e.g., half wavelength or less to avoid spatial aliasing or grating lobes) to facilitate beamforming. Second, the individual sub-arrays are-spaced far apart (e.g., 5-15 wavelengths) to obtain spatial diversity. The preferred embodiment uses each sub-array for beamforming for CCI suppression. Since the post-beamforming outputs from each sub-array may be largely affected by noise only, they can be diversity combined. Diversity combining can be done using MRC (which requires channel estimation). But channel estimation is expected to be easier in the second stage since the first stage is expected to greatly reduce the interference. No FEQ is required since the MRC essentially serves the role of a xe2x80x9cmulti-elementxe2x80x9d FEQ. If MRC is not feasible, switched diversity combining (SDC) may be used by measuring the instantaneous SNR for each sub-array""s output and selecting the xe2x80x9cbestxe2x80x9d option each symbol time (other variations are also possible). In this case, a standard FEQ is still required. As mentioned earlier, techniques for channel estimation via time-frequency patterns of pilots can be used in conjunction with the proposed AA architecture and an overall improvement in performance and/or reduction in complexity/bandwidth overhead of those algorithms is expected. Note that the use of this two-stage architecture and method is not limited to operation over FFC only and can be used over SFC as well instead of MSOC (but MSOC is very difficult to implement over FFC). Thus, the two-stage method can be considered to be more general. Nor is the number of elements in each sub-array or the number of sub-arrays limited. Increasing the number of elements in each sub-array can provide for more-optimal beamforming while increasing the number of sub-arrays outputs that are diversity combined can serve to further reduce interference. As an example, a preferred embodiment may have each sub-array made up of 4-8 elements with 2 sub-arrays for a total of 8-16 elements.
In addition, beamforming in stage 1 can be done according to any one of a number of criterion. The preferred embodiment uses direction-of-arrival (DOA) based constraints for beamforming. DOA based constraints may be used when signals are directional such as in rural or suburban environments, but are less desirable when the angle spread is large (such as in indoor environments).
A number of methods may be used for DOA estimation. For example, some remote transmitters may be equipped with GPS type equipment to enable the BS to compute this information. Other possible methods are the use of BS triangulation via time-difference-of-arrival (TDOA) measurements. One method to estimate DOA""s of a given user is by using adjacent antenna array elements in each sub-array. The idea here is to extract the phase differences between complex baseband (symbol rate) samples from adjacent sensors or doublets. As mentioned above, the sensors in each sub-array are spaced a half-wavelength apart or closer to avoid spatial aliasing. For a range of channel scenarios, the fading experienced by adjacent sensors is almost perfectly correlated. For the range of signal bandwidths and RF carrier frequencies, the signals can be considered to be narrowband. Thus, when the signals are coherently downconverted and demodulated, a mutual phase offset is induced between the samples obtained from adjacent sensors. This phase offset is proportional to the inter-sensor spacing (normalized in wavelengths) and the sine of the DOA measured with respect to the normal to the array. This spatially induced phase offset is not only obtained for successive symbols, but also for all bins being used by a particular user. Thus, the measured phase offset can be smoothed in space (over multiple doublets in each sub-array and using multiple sub-arrays), in time (over a block of symbols), and in frequency (over multiple bins used by the same user) to mitigate the effect of noise. Note that since all array processing algorithms operate on complex baseband outputs, the processing can be efficiently done in the frequency domain. Also note that the AA receiver structures may also be implemented in conjunction with sectorized cells. For example, each cell can have 3-6 sectors and an AA receiver can be used within each sector. Of course, one of the advantages of AA in cellular/PCS systems is to achieve a reduction in the number of sectors per cell (which will improve the trunking efficiency) and still derive benefits of spatial separation between signals.
Generally, according to the preferred embodiment, the BS attempts to allocate bins to facilitate or augment the mitigation of IBI and CCI. In this method, the BS continually monitors a number of parameters and uses them to compute the bin allocations for a given user. Such allocations are typically made at start-up, but may also be made on-the-fly for non-constant bit-rate type applications. The allocations can also be changed dynamically in response to changes in the prevalent noise and interference conditions. For operation over SFC, deep fades may occur over portions of the signal spectrum (perhaps spanning several bins) for extended periods of time, perhaps seconds or even minutes. Bins can also be dynamically reassigned in such cases.
To be more specific, consider a system where signals are spatially localized (e.g. most cellular/PCS rural/suburban systems). Assume that the BS has estimates of the direction and received power of the dominant signal paths of all active users. Due to multipath communications, each user may have more than one distinct (and strong) multipath directions. The BS will typically set a limit on the number of dominant paths that it can take into account (such as 2 or 3) due to constraints in computation/memory etc. Such estimates can be computed as per discussion above or by pilots embedded in the sub-symbol streams of users. Similarly, the BS also computes the power and directions of CCI across the band of interest. For example, if the two stage receiver architecture described above is used, these directions and powers can be obtained from the beamforming coefficients used in each sub-array.
For example, assume that a particular user is to be allocated K bins. Using these inputs, the BS allocates bins to satisfy the following (desired) criteria:
1. The K bins belonging to any one user should be spaced as far apart in frequency as possible to minimize mutual IBI. Spacing the bins belonging to each user over a wide range of frequencies within the band also provides frequency diversity. Frequency diversity is desirable because it serves to lessen the effects of fading over a certain frequency range. For example, by allocating many widely spaced frequency bins to a single user, if the operating environment is such that some of the bins experience fading, the overall signal quality will still remain high because the other user bins will not experience this fading; in short, fading over a small frequency range within the band will not effect the whole signal.
2. Each bin is placed in a neighborhood with bins belonging to other users which are spaced as far apart as possible in the dominant DOAs of their signals. For example, a 3-5 bin neighborhood is expected to be suitable for most applications.
3. Each bin is placed in a neighborhood with bins belonging to other users such that differences in signal strength of active bins in the neighborhood are minimized.
4. Each bin is placed in a spectral location such that there are no co-channel interferers in the same frequency band. If no such locations are available, spectral locations are chosen based on the DOA of the CCI and the signal strength of the CCI. In general, CCI bins with lower CCI signal strength are assigned before bins with higher CCI signal strength. Also, bins are allocated so that the difference in the DOA""s of the particular user and the CCI are as large as possible. These criteria are balanced depending upon the operating environment.
These criterion lead to better separation of potentially interfering signals in the spatial domain, thus facilitating the operation of spatial interference suppression techniques. The above criteria may be xe2x80x9cweightedxe2x80x9d differently to construct algorithms or flowcharts optimized for a specific (or category of) channel and interference scenarios, as will be apparent to those skilled in the art. For example. If IBI is the dominant impairment, items 1 and 2 above can be given the most importance. Item 3 is important if the system is operated in an environment with a wide range of received power levels. Similarly, if CCI is the dominant impairment, item 4 is given the highest priority. Since the number of bins in an OFDM can be quite large (several hundred bins is common), the implementation of the overall algorithm must be reasonably simple to enable execution in real-time. For instance, one possible way to implement it would be to construct a data structure which contains a table of information about each bin. This would include information such as whether the bin is active or inactive (at any given time), the user occupying the bin if any, whether it is a data, control or pilot bin, modulation scheme and constellation size of sub-symbols in the bin, received power level, dominant DOA""s of the user occupying the bin, power level and DOA""s of any co-channel interferers spectrally overlapping with the bin, etc. Note that all items of the above information may not be available or be able to be computed for each bin at all times. Certain of the above factors (like received power levels) appear to be best suited for update on a periodic (scheduled) basis (such as every n milliseconds or during each frame as per some existing framing structure). Other items are better suited for update in an event driven mode (such as user activity and constellation size), i.e., when a user arrives, departs, requests (or is forced to have) a change in the amount of allocated bandwidth.
Turning now to a more rigorous examination of uplink multiple access using OFDM. In an OFDMA system, the entire uplink bandwidth processed by a base-station is (dynamically) allocated among a group of users. While the downlink is always synchronous, unlike uplink TDMA systems in which remote units transmit in bursts in specified periodic time-slots, uplink OFDMA can be made synchronous using the method of loop-timing. In this method, each mobile transceiver first synchronizes itself to the base-station on the downlink and then derives its uplink transmitter timing reference from the recovered downlink clock. To facilitate the former task, the base-station embeds pilot tones in the transmitted downlink signal which are utilized by the remote receiver to xe2x80x9clock-onxe2x80x9d to the base""s timing reference. To overcome frequency selective fading across the signal bandwidth, multiple pilots can be used. While conventional baseband digital phase locked loops can be used for operation over slowly time-varying channels, for frequency acquisition and tracking algorithms suitable for operation over fast time-varying channels. The local timing reference for mobile transceivers are usually derived from a Voltage Controlled Crystal Oscillator (VCXO) which provides the timing reference for the receiver A/D, transmitter D/A and all radio frequency (RF) circuitry. Frequency offsets between the receive and transmit symbol clock occur due to non-idealities in the remote transceiver VCXOs, possibly of the order of several parts-per-million (ppm).
Assuming that the initial tasks of carrier frequency synchronization, symbol timing recovery and symbol time alignment have been completed this enables the base-station receiver to demodulate received baseband signals from all users with a single FFT. The base-station is also responsible for all bandwidth management functions to provide each unit with shared access to the uplink channel.